1. Field of the Invention
Applicants' invention relates to devices which serve as antennas to receive and transmit electromagnetic energy. More specifically, the present invention relates to the use of antennas for direction-finding systems. The antennas are fabricated from non-metallic, polymer, composite materials rather than metals, which provide certain advantages, particularly with respect to being more difficult to detect by radar and being lighter weight. Also, there is an ability to tailor the electrical conductivity and the radar reflectivity of the composite materials.
2. Background Information
Presently, antennas are used in many configurations and with many different electrical connections to receive and transmit electromagnetic energy. Some antennas are very simple, such as car radios; but other antennas have large numbers of antenna elements in complex geometric arrays with very complex electrical connections between the elements. A common feature of the antennas is they conduct electrical signals and currents so that electromagnetic energy is transmitted and/or received. This requires that the materials of the antennas, especially the antenna elements, be electrically conductive, which has required in the past that they be fabricated from metals. A minimum requirement is that the material of the antenna elements have sufficient electrical conductivity for the antenna to receive or transmit electromagnetic energy efficiently. Furthermore, the material of the supporting structure for the antenna elements should have sufficiently high electrical conductivity to provide shielding for electronics within the structure and to provide electrical symmetry.
Radio direction-finding (DF) is the process of electronically determining the direction of arrival of a radio signal transmission. The techniques for obtaining cross bearings of an emitter and using triangulation to estimate target positions are well-known. The ability to ascertain the geographical location of an emitting transmitter offers important capabilities for many modem communications applications, such as land, air, and sea rescue, duress alarm and location, law enforcement, and military intelligence. There are numerous direction-finding antennas and systems in the prior art.
Some receiving antennas can be used for radio direction-finding purposes. There are a number of types of antenna elements which are positioned with respect to each other in different configurations. Examples of types of antenna elements include monopoles, dipoles, simple loops, and ferrite loops. Configurations include Adcocks, dipole Adcocks, quadruple Adcocks, Rocke Adcocks, spaced loops, crossed loops, Breuningers, and doppler arrays. Also, the antenna configuration can be rotating or nonrotating and fixed or mobile.
Typically, direction-finding antennas are mounted high and/or on the external surfaces of objects so they have unobstructed views of the arriving electromagnetic energy. This is especially the case when the platform on which they are mounted is a ship, airplane, land vehicle, or building.
In the past, the requirement of high electrical conductivity of materials from which antennas were fabricated necessitated that metals be used. Now, polymeric materials with sufficiently high electrical conductivities along with other desirable properties have been developed and are commercially available. A polymer is a large molecule built up by the repetition of small, simple chemical units. The repeat unit is usually equivalent to the monomer, or starting material from which the polymer is formed. Polymers are mostly compounds of carbon, hydrogen, nitrogen, and oxygen and generally melt or decompose at high temperatures. The quantity of basic polymer materials is large with variations and modifications increasing the number considerably.
Recent work with electrically-conductive polymer materials has resulted in increased electrical conductivities and improved processabilities and environmental stabilities. The conductive polymers can be applied as coatings on other material filaments, for example, which can then be woven into fabrics. Several major classes of electrically conductive materials now exist, including polyacetylene, polyheterocycles (polypyrole and polycarbazole), and poly (arylene vinylenes). Fiberglass filaments which were coated with polypyrole and then woven into fabrics were used here. The electrical conductivity level can be varied significantly as a function of the dopant level in the conductive materials. These conductive materials are used with composite polymers.
Composite materials are composed of at least two different constituents to obtain the optimum combination of properties of the individual elements. The constituents may be metals, polymers, ceramics, glasses, cements, and other materials. Often a composite material is created from fiber or cloth, which serves as a reinforcement, in a matrix of another material. The reinforcement provides strength properties (carries the load, provides stiffness, etc.) of the composite while the matrix holds the reinforcement in place, distributes or transfers loads applied to the composite, and protects the reinforcement from damage and environmental degradation. The matrix can be a plastic, metal, ceramic, glass, etc. in which the reinforcement is embedded. Thermosetting polymers are used often for the matrices and include epoxy, polyester, phenolic, and polyimide resins. The reinforcement can range from flakes or short fibers through complex textile forms. The constituents of a composite material do not dissolve or merge completely, so that they retain their identity as they act together.
An inherent disadvantage of conventional metal direction-finding antennas is they have high reflections of radar signals. For certain conditions, such as during a war, stealth or low observability characteristics are very important. Determining the radar cross section (RCS) of objects is another way to characterize and compare radar reflections from objects. RCS is a measure of the size of a target observed by radar. It is expressed in decibels and referenced to a standard target. RCS is a function of the physical cross section area, shape, material, and orientation of the target and the frequency and polarization of the incident energy.
Because antennas fabricated from fiberglass composite materials have much lower radar reflections, and much lower radar cross sections, they have significant advantages when that is important. When low radar observability of a direction-finding antenna is desired, a good material characteristic is high electrical conductivity at the DF frequencies of interest, about 30 megaHertz (MHz) to about 1 gigaHertz (GHz), with low conductivity at radar frequencies, generally well above 1 GHz. In addition, the weights of polymer composite antennas may be significantly less, which is very important for certain applications.